Prior Art
Many types of electro-optical devices are used to detect and image a scene. The scene radiates energy by self emission, as in the case of thermal radiation, or it can reflect radiation, as in the case of reflected sunlight, or it can radiate and reflect simultaneously. In any case, radiation in the form of photons or electro-magnetic radiation from a scene is directed to a lens which focuses the photons onto a detector or an array of detectors. The lens and detector are matched for the passband of radiation of interest and a system designed for operation in a particular part of the spectrum uses a detector that responds to photos in the same part of the spectrum. The early development of electro-optical systems has been concerned mainly with the detector and a means to read the detected signal. Television camera systems use photocathodes to capture scene photons and electronic beam scanning to read the photocathodes whereas image intensifier (I.sup.2) systems use photocathodes and either electron-optics to accelerate the photoelectrons from the photocathode microchannel plates (MCP) to amplify the photoelectrons. Thermal imaging systems use photon or thermal detector which absorb thermal photons. Photon detectors absorb thermal photons and convert them into electron-hole pairs whereas thermal detectors absorb thermal photons and convert them into temperature changes in the detector. Thermal systems use various types of scanning schemes, such as electron-beam, mechanical or electronic scanning.
In principle, image intensifier systems are the simplest type of image converters as they do not require any type of scanning. Accordingly, they are referred to as direct view devices, which depend on photoelectrons from photocathodes to convert images. Photocathodes have limited spectral response characteristics and require an ambient light level to function. The subject invention provides for an extended spectral response for I.sup.2 devices by using detectors that have extended spectral responses in lieu of the regular photocathode. Thus, the simplicity of an I.sup.2 system may be applied to imaging systems that respond in various parts of the electro-magnetic spectrum, but especially in the thermal infrared band.
The technology of cold cathode emission has been used for imaging purposes and uses tunnel electrons that tunnel through a metallic surface under the influence of a strong electric field. The electric field lowers the surface potential barrier of the metal allowing electrons to tunnel their way through the surface. The resulting current from the tunnel electron phenomenon depends on the metal's work function and the applied electric field with the tunnel current being highly non-linear. If an array of flat cold cathode electrodes be used for imaging purposes, the field and work function must be tightly controlled to prevent image non-uniformity or fixed pattern noise. In addition, since the cold cathode emission process is so non-linear, a very small change in the applied voltage produces a large change in the tunnel current which limits the usefulness of a cold cathode emission array from a contrast viewpoint, as small changes in contrast require microscopic changes in voltage which is extremely difficult to achieve.